It's very difficult/impossible to get a perfect mixture with a single carb on our bikes. Mostly it's because of the way our valve train is set up. The crank on the XS400 is offset 180 degrees causing both intake valves to be open for a very short time in unison (overlap). This causes a huge draw of air through the single carb that no amount of jetting can compensate for, causing a very lean condition for the second cylinder to perform its intake stroke.
Compare that to the XS650, with a 360 degree offset, which has no problem working with a single carb because there's no overlap. In my opinion if you wanted to seriously move to a single carb you would have to get a crankshaft that fit our bike with a 360 degree offset,a new cam that would match, and mess with your timing to spark the second cylinder at your new top-dead-center. All of which I don't think is worth sinking your money into.